Low-dimensional materials such as nanocrystals, nanowires, and nanotubes are at the forefront of materials research. Novel physical and chemical phenomena observed in these systems allow us to envision a variety of next-generation technologies such as high-performance transistors for nano- and microelectronics, low-cost, high-efficiency photovoltaics, high-density magnetic storage media, nano-electromechanical systems and miniaturized biosensors. Fundamental challenges include synthesis of chemically and structurally well-defined nanoscale materials, developing methods of assembly, and establishing ...